According to this thread, the Roku LT and many other Roku models use an IR protocol closely related to NEC. The device numbers are 234.194. Some of the latest OneForAll remotes carry the exact Roku IR protocol with setup code Cable/STB 3061 (PID 021A), but even the latest version of the 7140 (there have been 4!) doesn't have this setup code. Nor does it have a setup code with NEC 234.194. All of this means that you can't program your 7140 from the keypad to work with the Roku.

2014 was the first year I saw a Roku code in a UEI remote, and it's setup code 03061 (CBL 3061). I'm afraid it doesn't exist on yours. You'll have to get a JP1 cable or borrow a Roku remote to learn from. If you'd rather buy a UEI remote that has it, the Inteset INT-422 would be a good choice.